The invention relates to battery circuits for electret microphones having uniform performance.
Electret microphones are widely used today in voice communications for their small size, light weight and high output. These microphones require a DC supply in order to operate because of an internal JFET circuit common to all electrets. Also available are the conventional dynamic microphones which are larger, heavier, with much lower output levels but require no external DC power.
In "integrated" or built-in applications such as corded, cordless and cellular telephones, electret mic bias is provided internally without difficulty. Stand-alone desk microphones or headsets are frequently used with telephones, PC sound cards or DVD (Digital Voice over Data) modems. There may be no provision to obtain DC bias from the input of the equipment that the microphone is connected to. If DC bias is an option, it will require reference to a technical manual and opening of the cabinet in order to move a switch or jumper to enable the bias path. Many consumers are reluctant to go inside the equipment and prefer a microphone solution that is ready to use.
A schematic diagram of a simple "battery box" circuit 100 for electret microphone operation is shown in FIG. 1. The circuit includes an electret mic 102 having an electret element 104 and a JFET transistor 106. A resistor 108 provides a DC path for the JFET drain current and isolates the audio output from the very low impedance of battery 110. The JFET drain terminal has a very high output impedance, so the resistor 108 brings the output down to a known, lower level typically 1-5 k.OMEGA.. The range of currents for the chosen electret element 104 will determine the required battery voltage, such that after the voltage drop through the resistor, enough voltage remains at the drain for the JFET to operate in the active region. Typical operating current for a Gentex Model 3065 electret microphone is 100-300 .mu.A with R at 2 k.OMEGA. from a 3V battery.
A capacitor 112 separates the DC path from the input of audio amplifier circuitry 114 and must be large enough for the input impedance of the amplifier and the lowest audio frequency of interest. For example, 1 .mu.F will high pass 100 Hz into 1500 .OMEGA. at -3 dB.
As long as the battery voltage in FIG. 1 is sufficient to keep the FET in the active region, normal operation will continue. The user, however, has no indication or knowledge of battery condition and eventually reduced voltage will cause FET saturation, lower output level and audio distortion. Before the condition is severe enough to be recognized by the user, speech intelligibility to human listeners and speech recognition by software programs will be adversely affected. Adaptive software which "learns better as it goes" will begin storing distorted data at low battery voltages and when the batteries are replaced, the user may sound "foreign" relative to the stored information. If the circuit of FIG. 1 were connected to equipment configured for dynamic microphones only, the much higher output level from the electret mic could cause overload and distortion.